Coriolis mass flowmeter and method of making same

ABSTRACT

A Coriolis mass flowmeter of a type in which a fluid to be measured is caused to flow in series by forming two parallel flow tubes  1  and  2,  which are obtained by bending one conduit. The inlet portion of conduit between an external connecting portion  35  connected to inlet piping and the inlet of one flow tube  2  is bent in the shape of the letter L and bent in another direction in combination, and the outlet portion of conduit between an external connecting portion  36  connected to outlet piping and the outlet of the other flow tube  1  is bent in the shape of the letter L symmetrically with respect to the above inlet portion of conduit and bent in another direction in combination. The external connecting portions  35  and  36  are arranged on the same axis. The present invention simplifies the construction of connection to external piping, simplifies the assembling of the Coriolis mass flowmeter itself, and permits a high-accuracy measurement of mass flow rate by reducing the vibrations of external piping, etc.

FIELD OF THE INVENTION

The present invention relates to a Coriolis mass flowmeter and, moreparticularly, to a loop-type Coriolis mass flowmeter in which twoparallel bent tubes are connected in series.

BACKGROUND OF THE INVENTION

When a tube through which a fluid to be measured flows is supported atthe one or both ends thereof and vibrated at this supporting point in adirection vertical to the direction of fluid flow in the tube, Coriolisforces acting on the tube (a tube to which vibrations are applied ishereinafter referred to as a flow tube) are proportional to a mass flowrate. A mass flowmeter based on this principle (a Coriolis massflowmeter) is well known. Flow tubes used in this Coriolis massflowmeter are broadly classified into bent tubes and straight tubesaccording to their shape.

In a Coriolis mass flowmeter of the straight tube type, a mass flow rateis measured as a difference in the displacement of the straight tubecaused by the Coriolis forces between the supporting point and themiddle portion of the straight tube, i.e., a phase difference signal,when the Coriolis mass flowmeter is vibrated in a direction vertical tothe axis of the straight tube supported at both ends. In such Coriolismass flow meters of the straight tube type, it is difficult to achievehigh detection sensitivity though they are simple, compact and robust inconstruction.

In contrast, Coriolis mass flowmeters of the bent tube type, in whichthe shape of the bent tube can be selected appropriately to effectivelytake out Coriolis forces, can detect ass flow rates with highsensitivity. It is also known that in order to more efficiently drivethis measuring bent tube, the bent tube in which a fluid to be measuredflows is constituted of two parallel tubes.

FIG. 4 schematically shows this conventional Coriolis mass flowmeter ofthe two parallel bent tube type. As shown in the figure, the flow tubeis fabricated from two parallel U-shaped tubes in which a branchingportion is formed on the inlet side of the fluid to be measured and ajunction portion is formed on the outlet side thereof. The fluid to bemeasured is equally divided into two flow tubes on the inlet side andjoins on the outlet side of the flow tubes. By causing the fluid to bemeasured to flow equally in two flows in this manner, it is possible toconstantly make the natural frequency of two flow tubes equal even whenthe kind of fluid changes, or the temperature fluctuates. It is knownthat this permits efficient and stable driving of flow tubes and makesit possible to form a Coriolis mass flowmeter that is not affected byexternal vibrations or temperature changes.

Pressure losses or clogging of flow tubes with the fluid to be measuredmay sometimes occur in the branching portion at the inlet of the fluidand the junction portion at the outlet thereof. This poses a problemespecially in the case of high-viscosity fluids and liquids such asperishable and easy-to-clog foods. Furthermore, when one flow tube isrecovered from clogging during the flushing of flow tubes with abranching portion, the flushing performance of the other flow tubedeteriorates, leading to a long flushing time.

A loop-type Coriolis mass flowmeter, in which two practically parallelbent tubes are fabricated from one bent tube as shown in FIG. 5 andthereby the fluid to be measured is caused to flow in series, is alsoknown as an improvement on the above Coriolis mass flowmeter.

In tubes of this loop type, however, the problem resulting from theformation of the above branching and junction portions can be solved,but their connection to external piping is complex and difficult becauseof their three-dimensional (not two-dimensional) geometry. At the sametime, measuring accuracy is affected by strains caused during thefabrication of flow tubes, and the tubes of the loop type aresusceptible to the effect of external vibrations.

SUMMARY OF THE INVENTION

To solve the above problems, an object of the present invention is toprovide a Coriolis mass flowmeter which has a simple connection toexternal piping and can be assembled in a simplified manner whilemaintaining the advantage of the Coriolis mass flowmeter having aparallel double bent tube, that is, freedom of pressure losses orclogging with a fluid by bending a single tube in the above-mentionedmanner to eliminate branching and junction portions.

Another object of the present invention is to provide a Coriolis massflowmeter which can measure mass flow rate with high accuracy byreducing strains caused during working and at the same time by reducingthe vibrations of external piping, etc.

The Coriolis mass flowmeter according to the present invention is a typein which a fluid to be measured is caused to flow in flow tubesconnected in series by forming two parallel flow tubes 1 and 2, whichare obtained by bending a single tube. The Coriolis mass flowmeter ischaracterized in that the inlet portion of conduit between an externalconnecting portion 35 connected to inlet piping and an inlet of one flowtube 2 is bent in an L shape and further bent in another direction, andthe outlet portion of conduit between an external connecting portion 36connected to outlet piping and an outlet of the other flow tube 1 bentin an L shape symmetrically with respect to the inlet portion ofconduit, with an additional bend in another direction combined, with theresult that the external connecting portions 35 and 36 are arranged onthe same axis. This facilitates horizontal external connection duringassembly. As mentioned above, loop-type bent tubes need to bethree-dimensionally bent. In the present invention, thisthree-dimensional bending can be accomplished with the connectingportions of the inlet portion of conduit, outlet portion of conduit andtwo flow tubes. Thus, flow tubes that require working without generationof strains can be formed by two-dimensional bending alone.

A fixing member 32 of hollow construction is provided to integrally fixeach end of the two flow tubes 1 and 2 via a supporting member 33fabricated from a separate sheet and there is also provided an outercasing 30 which further supports this fixing member 32. The supportingmember 33 is fixedly fitted to the fixing member at two locationsopposite to the hollow fixing member 32 for each end of each of the flowtubes 1 and 2 and, at the same time, fixes the ends of the flow tubes 1and 2. This facilitates the installation and fixing of the flow tubesand can prevent unwanted torque, vibration and wobble of the flow tubes.In the present invention, firm supporting is achieved at low cost withthe use of supporting members fabricated from sheets and a hollow fixingmember for fixing the supporting members, thereby reducing thevibrations of external piping, etc. and permitting a high-accuracymeasurement of mass flow rate.

The outer casing 30 has a hollow shell construction and the fixingmember 32 is supported by the outer casing at a plurality of points(supporting portions 38) independently, not over the whole surface.Furthermore, a pressure-resistant case 31 is provided to be tightlyconnected to the outer casing 30. This pressure-resistant case 31accommodates one conduit including the flow tubes and all accessorycomponents of the conduit that include a drive 15, a pair of detectingsensors 16 and 17 and the fixing member 32. In the present invention,the outer casing has a shell construction and the pressure-resistantcase is connected integrally to the outer casing. This not only enablesthe vibrating structural portions of flow tubes to be strongly protectedagainst stresses from external piping, but also prevents the liquidflowing through the tubes from flowing out of the pressure-resistantvessel even in case of a breakage of the tubes.

Furthermore, the method of making the Coriolis mass flowmeter accordingto the present invention involves integrally assembling the conduit andaccessory components of the conduit, such as the fixing member 32 forfixing each end of the flow tubes, the drive 15 and the pair ofdetecting sensors 16 and 17, as a unit. This unit is arranged in theouter casing 30 having a shell construction in a manner that the outerconnecting portions 35 and 36 are on the same axis. This facilitatesassembling and can reduce the assembling time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a front view of an example of Coriolis mass flowmeter towhich the present invention is applied.

FIG. 2 shows the construction of the Coriolis mass flowmeter shown inFIG. 1 and in particular of the flow tubes as seen from the top.

FIG. 3 shows Part A of FIG. 1 in detail.

FIG. 4 schematically shows a conventional Coriolis mass flowmeter of theparallel two bent tubes.

FIG. 5 schematically shows a conventional loop-type Coriolis massflowmeter in which a fluid to be measured is caused to flow in series.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 3 show an example of Coriolis mass flowmeter to which thepresent invention is applied. The following explanation will be made bysupposing a case where flow tubes of the two parallel bent tube typefabricated by bending one tube are installed in a vertical plane.However, it is also possible to install the flow tubes in a horizontalplane. FIG. 1 is a front view of the flow tubes. FIG. 2 is a view partlyin section of the Coriolis mass flowmeter installed in a vertical planeas viewed from the top (the top of FIG. 1) in order to show theconstruction of the flow tubes in particular. In FIG. 2, the drive,detecting sensors, etc. are not shown. FIG. 3 is a detail of Part A ofFIG. 1.

The flow tubes 1 and 2 of the Coriolis mass flowmeter are bent tubescurved in a portal shape and both the tubes have the same shape. Theseflow tubes are fabricated from one continuous conduit. As mentionedabove, flow tubes are portions of tube driven for resonance to produceCoriolis forces and they are expressed by flow tube 1 and flow tube 2.The whole of the flow tubes, including connecting portions which connectthese two flow tubes together, and an inlet portion and an outletportion connected to outer piping, is fabricated from one conduit. ThisCoriolis mass flowmeter is horizontally symmetric and permits a liquidto flow in and out from either side. However, the following descriptionis based on the assumption that in the Coriolis mass flowmeter shown, afluid to be measured flows in from the left side of FIG. 1 (FIG. 2) andflows out to the right side of the figure.

A liquid to be measured flows horizontally into an inlet portion ofconduit after passing through an external connecting portion 35 fromexternal inlet piping. The inlet portion of conduit is bent upward inthe shape of the letter L as shown in FIG. 1 and is also bent in adifferent direction as shown in FIG. 2. The liquid to be measured passesthrough a flow tube 2 from this inlet portion of conduit, enters a flowtube 1 after passing through the connecting portion of conduit from thebottom on the right side of FIG. 1, and further passes through theoutlet portion of conduit from the flow tube 1, finally flowing out intoexternal outlet piping from an external connecting portion 36. Theoutlet portion of conduit is bent and fabricated symmetrically withrespect to the inlet portion of conduit.

The conduit is thus formed in a loop shape and bent in an L shape to theexternal connecting portions 35 and 36 to inlet piping/outlet piping inthe directions opposite to each other. At the same time, bending in adifferent direction is performed and the external connecting portions toinlet piping and outlet piping are arranged on the same axis, thusfacilitating external horizontal connection during assembling.

The flow tubes 1 and 2 are provided with a drive 15 and a pair ofdetecting sensors 16 and 17, which will be described later in detail,and the drive and detecting sensors are wired. Near both ends of theflow tubes 1 and 2 are provided bases plates 26 and 27 for forming thenodes of vibrations when the flow tubes 1 and 2 are driven like a tuningfork. These bases plates 26 and 27 are secured to each of the flow tubes1 and 2 so that the flow tubes 1 and 2 are kept parallel.

A supporting member 33 for fixing each end of the flow tubes 1 and 2 isfabricated from a sheet as seen in FIG. 3, which shows a detail of PartA of FIG. 1. The inlet portion and outlet portion are fixed by means ofdifferent supporting members 33 for each flow tube. The installation andfixing of the flow tubes are made easy by using different supportingmembers 33. Furthermore, thermal energy can be reduced to a minimumnecessary amount in vacuum-brazing the flow tubes to the supportingmembers 33 by using members of small thermal capacity.

This supporting member 33 is connected to the fixing member 32 of hollowconstruction in at least two upper and lower places. The illustratedfixing member 32, which is formed as a hollow member having an almostrectangular shape in cross section, fixes one end of the flow tube withboth of the upper and lower sides of the rectangle, respectively. As aconsequence, each end of one flow tube is supported in two places on thetwo sides of the hollow rectangle, and other two sides forming the sidesof the fixing member 32 of hollow rectangle are fixed to an outer casing30 as will be described later. In general, a bar or pipe that vibrateslike a cantilever must be secured with a certain width. For example, iffixed by a block-like supporting member, a vibrating tube receivesvarious stresses from the supporting member due to a difference inthermal capacity. An ideal type of supporting is supporting at twopints. This gives only accurate positions of support and does notproduce an unnecessary effect. Therefore, the connection in at least twoupper and lower places can prevent inconvenient torques, vibrations andshakes of the conduit.

The same material as that for the thin-walled flow tubes can be used forthe supporting members 33, thereby enabling the penetration ofimpurities into the thin-walled flow tubes during securing by brazing tobe prevented and various problems arising from the expansion andshrinkage due to a difference in thermal capacity to be avoided.Materials usually adopted in this technical field, such as stainlesssteel, Hastelloy and titanium alloys, can be used as the material forthe flow tubes 1 and 2.

A material having a predetermined strength can be used as the materialfor the fixing member 32. Furthermore, unlike the supporting member 33that is generally expensive because of the use of the same material asthat for the flow tubes, cost for the fixing member 32 can be saved byusing an inexpensive material that does not require corrosionresistance, for example. In other words, though the fixing by the fixingmember 32 must be firm, the use of a hollow structural member canachieve firm two-point supporting, provides a small thermal capacity,and allows the performance as a flowmeter to be improved due to theavoidance of the problem during brazing, a reduction of cost and animprovement in temperature responsiveness. The securing of thesupporting member 33 to the fixing member 32 can be performed, as shownin FIG. 3, by preparing a corresponding cut away portion in the fixingmember 32 with a size a little smaller than the supporting member 33,superposing the supporting member 33 on this cut away portion, andsecuring the surrounding area using an appropriate securing means suchas brazing and welding.

The illustrated Coriolis mass flowmeter is provided with an outer casing30 having a hollow shell construction (a curved construction formed by asheet which is thin compared with the size of the structure). This outercasing has an inlet portion and an outlet portion, which are providedwith a hollow cylindrical member 40 and a hollow cylindrical member 41,respectively. In assembling the outer casing 30, the hollow cylindricalmembers 40 and 41 are inserted from outside after passing externalconnecting portions 35 and 36 of the conduit portion through the inletand outlet portions of the outer casing, respectively, and gaps betweenthe outer connecting portions 35 and 36 and the hollow cylindricalmembers 40 and 41, and gaps between the hollow cylindrical members 40and 41 and the outer casing 30 are secured by welding and the like. Thispermits mechanical fixing and, at the same time, can shut off theinterior space from outside in conjunction with a pressure-resistantcase 31. Because the outer casing 30 has a hollow shell construction foraccommodating the conduit portion, the installation of the conduitportion can be easily and positively carried out, making it easy to forman integrated construction. Furthermore, this can strongly protect thevibrating structural portions of flow tubes against the stresses fromouter piping. The term “conduit portion” used in the presentspecification refers to one conduit portion including the flow tubes andaccessory components of the conduit that include the drive 15, the pairof detecting sensors 16 and 17 and the fixing member 32.

In integrating the construction of the conduit portion as a unit,incorporating the unit into the outer casing 30 of shell constructionand connecting it, the feature of the present invention that the shapeof the outlet portion/inlet portion of conduit is the shape of theletter L makes it possible to incorporate and install the unit withinthe outer casing 30 with a length a little smaller than that of the unitafter the assembling of the unit. Thus, the construction of the conduitportion as a unit makes assembling easy and can reduce the assemblingtime.

In order to ensure that the external vibrations of piping, etc. can bereduced, the connection of the fixing member 32 to the outer casing 30is performed not over the whole surface of the side of fixing member 32,that is, it is performed in spots rather than on the surface. Thisconnection is effected by securing four independent supporting portions38 as shown in FIG. 2 to both of the fixing member 32 and outer casing30, for example, by welding. This can reduce external vibrations (ofpiping system, self-oscillation system, etc.). After this assembling,the pressure-resistant case 31 is tightly connected integrally to theouter casing 30 by an appropriate means such as welding and screwing toform a pressure resistant vessel. This prevents the fluid flowingthrough the tubes from flowing out of the pressure-resistant vessel evenif the tubes should be broken.

The drive 15 that drives the flow tubes 1 and 2 of such two parallelbent tubes is usually composed of a coil and a magnet. The coil of thedrive is mounted on one of the two flow tubes 1 and 2, and the magnet ismounted on the other flow tube. The coil and the magnet drive the twoflow tubes 1 and 2 with resonance in reverse phases relative to eachother. The pair of vibration detecting sensors 16 and 17, each composedof a coil and a magnet, is installed in the positions horizontallysymmetrical with respect to the installed position of the drive 15 todetect a difference in phase which is proportional to Coriolis forces.The coil and magnet of this sensor are also independently installed viafixtures; that is, the coil is mounted on one flow tube and the magnetis mounted on the other flow tube.

As shown in FIG. 1, the wiring to the coil of the drive 15 is passedthrough a wiring lead-in portion 39 attached to the outer casing 30 fromoutside this Coriolis mass flowmeter, guided along a column 10 andconnected from the end of the column via a flexible printed board 12.This lead-in portion 39 shuts off the internal space and external spacealthough it allows the wiring to pass through. The flexible printedboard itself is known and a flexible printed board of prescribed widthin which copper foil for wiring is sandwiched with polyimide films canbe used.

The wiring of the pair of sensors 16 and 17 is passed from outside tothe end of the column 10 in the same manner as with the wiring to thedrive 15. From the end of the column 10, the wiring is then guided viaanother flexible printed board disposed opposite to the above flexibleprinted board 12 and then via Teflon wires (copper wires or copper foilsheathed with Teflon) which are guided on the surface of a Teflon tubein the two right and left directions.

The column 10 is thus mounted to the fixing member 32 in a manner thatthe front face of the column 10 is opposed to the drive 15 to effect thewiring to the coils of the drive and the detecting sensors. Furthermore,the column 10 can also support the wiring to a temperature detectingsensor 20.

When a fluid to be measured flows through a Coriolis mass flowmeter thusfabricated, a flow rate measurement is made in a usual manner. The drive15 installed in the middle portion of the flow tubes 1 and 2 drives withresonance one flow tube in a reverse phase relative to the other flowtube in a direction vertical to the plane where the flow tubes 1 and 2are present. A difference in phase due to Coriolis forces based on theresonance is detected by the pair of vibration detecting sensors 16 and17 installed between the fixed ends and middle of the flow tubes 1 and2.

The present invention is not limited to flow tubes of the above portaltype and can be applied to loop-type Coriolis mass flowmeters using benttubes with any shape such as a circle and the letter U, for example.

Industrial Applicability

As mentioned above, the present invention can provide a Coriolis massflowmeter which has a simple construction of connection to externalpiping and can be assembled in a simplified manner and in which the flowtubes themselves need not to be three-dimensionally bent whilemaintaining the advantage of the Coriolis mass flowmeter containing aparallel double bent tube formed by bending one tube that pressurelosses or clogging with a fluid do not occur because there is nobranching portion or junction portion.

What is claimed is:
 1. A Coriolis mass flowmeter comprising: a conduitstructure fabricated from one bent conduit with an inlet portion, anoutlet portion, a first flow tube, a second flow tube parallel to saidfirst flow tube, said first flow tube and said second flow tube forapplying vibrations and detecting Coriolis forces based on the appliedvibrations, and a connecting portion connected to said first flow tubeand to said second flow tube; a first bases plate connected to each ofsaid first flow tube and second flow tube adjacent to a respective endof each of said first flow tube and second flow tube; a second basesplate connected to each of said first flow tube and second flow tubeadjacent to another respective end of each of the first flow tube andsecond flow tube, said first bases plate and said second bases plateforming nodes of vibration when the flow tubes are driven; a fixingmember for integrally fixing each respective end of said first flow tubeand said second flow tube; a first supporting member; a secondsupporting member; an outer casing, said fixing member being secured tosaid outer casing; an inlet external connecting portion for connectionto inlet piping; an outlet external connecting portion for connection tooutlet piping, said inlet portion of said conduit extending between saidinlet external connecting portion and an inlet of one of said flow tubesand being bent in an L shape and being further bent in anotherdirection, said outlet portion of said conduit extending between saidoutlet external connecting portion and an outlet of another of said flowtubes being bent in an L shape symmetrically with respect to said inletportion and being further bent in another direction, said inlet externalconnecting portion and said outlet external connecting portion beingarranged coaxially, said first supporting member including sheetsindividually fixing ends of said first flow tube, said second supportingmember including sheets individually fixing ends of said second flowtube, said first and second supporting members being each fixed by saidfixing member, said fixing member having a hollow structure and saidsupporting members being fixed to said fixing member at two oppositelocations of said hollow structure for each end of each of the flowtubes and fixes the ends of the flow tubes.
 2. A Coriolis mass flowmeteraccording to claim 1, wherein said outer casing has a hollow shellconstruction and is provided with an inlet portion of outer casing andan outlet portion of outer casing through which said first externalconnecting portion and said outlet external connecting portion ,respectively, are passed through and secured.
 3. A Coriolis massflowmeter according to claim 2, wherein said fixing member is supportedby said outer casing in a plurality of places independently, and notover the whole surface of said outer casing.
 4. A Coriolis massflowmeter according to claim 1 further comprising a pressure-resistantcase to be tightly connected to the outer casing, saidpressure-resistant case accommodating one conduit including said flowtubes and accessory components of the conduit, said accessory componentsincluding a drive, a pair of detecting sensors and said fixing member.5. A Coriolis mass flowmeter comprising: an integral conduit structurewith an inlet portion, an outlet portion, a first flow tube portionextending substantially in a first flow tube portion plane, a secondflow tube portion extending substantially in a second flow tube portionplane, said first flow tube portion plane, being substantially parallelto said second flow tube portion plane and a connecting portionconnecting said first flow tube portion to said second flow tubeportion; a first external connecting portion for connection to inletpiping; a second external connecting portion for connection to outletpiping, said inlet portion of said conduit extending between said firstexternal connecting portion and an inlet of said first flow tube portionand being bent in an L shape and being further bent in anotherdirection, said outlet portion of said conduit extending between saidsecond external connecting portion and an outlet of said second flowtube portion and being bent in an L shape symmetrically with respect tosaid inlet portion and being further bent in another direction, saidfirst external connecting portion and said second external connectingportion being arranged coaxially; a fixing member; an outer casing, saidfixing member being secured to said outer casing; first supportingmembers connected to said fixing members and including sheetsindividually fixing ends of said first flow tube portion; secondsupporting members connected to said fixing members and including sheetsindividually fixing ends of said second flow tube portion, said firstflow tube portion and said second flow tube portion extending from oneside of said fixing element, said connecting portion being connected tosaid first flow tube portion and said second flow tube portion andextending at another side of said fixing element, said inlet portionbeing connected to said first flow tube portion and extending at saidanother side of said fixing element and said outlet portion beingconnected to said second flow tube portion and extending at said anotherside of said fixing element; a first bases plate connected to each ofsaid first flow tube and second flow tube adjacent to a respective endof each of said first flow tube and second flow tube; a second basesplate connected to each of said first flow tube and second flow tubeadjacent to another respective end of each of the first flow tube andsecond flow tube; a drive connected to said first flow tube portion andconnected to said second flow tube portion for driving said first flowtube portion and said second flow tube portion relative to nodes ofvibration formed by said first bases plate and said second bases plate;and a pair of detecting sensors for detecting vibrations of said firstflow tube portion and said second flow tube portion.
 6. A Coriolis massflowmeter according to claim 5, wherein said fixing member has a hollowstructure and said supporting members are fixed to said fixing member atopposite locations of said hollow structure for each end of each of saidflow tubes to fix ends of said flow tubes.
 7. A Coriolis mass flowmeteraccording to claim 5, wherein said outer casing has a hollow shellconstruction and is provided with an inlet portion of outer casing andan outlet portion of outer casing through which said first externalconnecting portion and said first external connecting portion,respectively, are passed through and secured.
 8. A Coriolis massflowmeter according to claim 7, wherein said fixing member is supportedby said outer casing in a plurality of places independently, and notover the whole surface.
 9. A Coriolis mass flowmeter according to claim5 further comprising a pressure-resistant case to be tightly connectedto the outer casing, said pressure-resistant case accommodating oneconduit including said flow tubes and accessory components of saidconduit, said accessory components including said drive, said pair ofdetecting sensors and said fixing member.
 10. A method of making aCoriolis mass flowmeter with a conduit having an inlet portion, anoutlet portion, two parallel flow tubes for applying vibrations anddetecting Coriolis forces based on the applied vibrations, and aconnecting portion for connecting the two flow tubes, the methodcomprising the steps of: fabricating the conduit as a single bentconduit through which a fluid to be measured flows in series through thetwo flow tubes; integrally fixing each end of said two flow tubes bymeans of a fixing member; providing a first external connecting portionto be connected to inlet piping and a outlet external connecting portionto be connected to outlet piping; bending the inlet portion of conduitbetween the external connecting portion connected to inlet piping andthe inlet of one flow tube in an L shape and further bending the inletportion in another direction; bending the outlet portion of conduitbetween the external connecting portion connected to outlet piping andthe outlet of the other flow tube in an L shape symmetrically withrespect to the inlet portion of conduit and further bending the outletportion in another direction; providing a unit of accessory componentsincluding a fixing member for fixing each end of the flow tubes, a driveand a pair of detecting sensors; arranging the unit of accessorycomponents in an outer casing having a shell construction such that saidfirst external connecting portion and said outlet external connectingportion are on the same axis; providing a first supporting memberincluding sheets individually fixing ends of one of the flow tubes tothe fixing member; providing a second supporting member including sheetsindividually fixing ends of another of the flow tubes to the fixingmember, said first and second supporting members being each fixed by thefixing member, the fixing member having a hollow structure and thesupporting members being fixed to the fixing member at two oppositelocations of the hollow structure for each end of each of the flow tubesand fixes the ends of the flow tubes; providing a first bases plateconnected to each of said first flow tube and second flow tube adjacentto a respective end of each of said first flow tube and second flowtube; and providing a second bases plate connected to each of said firstflow tube and second flow tube adjacent to another respective end ofeach of the first flow tube and second flow tube such that said firstbases plate and said second bases plate form nodes of vibration when theflow tubes are driven.